a) Field of the Invention
This invention relates to a process and apparatus for the production of a webbing take-up spindle suitable for use in a webbing retractor of a seat belt system. The webbing take-up spindle includes, as integrally formed elements, a spindle portion for taking up a webbing and ratchet teeth for stopping the spindle portion when the retractor has been locked.
b) Description of the Related Art
A webbing take-up spindle comprises a spindle portion and ratchet teeth as described above. As a very large force is applied in the event of a collision, the webbing take-up spindle is required to have high strength. Therefore it is usually produced by cold forming.
Several processes are known as production processes making use of cold forming. The cold forming process, which is disclosed as one of such known processes in Japanese Patent Publication (Kokoku) No. HEI 5-13739 published Feb. 23, 1993, comprises the following steps as shown in FIGS. 5A to 5F.
First, a cylindrical first blank 31a shown in FIG. 5A is provided. This cylindrical first blank 31a has a volume needed to form a take-up spindle.
(1) First step: The first blank 31a is subjected to end face setting or sizing in a die so that a shallow conical recess is formed in one end and a conical taper is formed on an opposite end. As a result, a Second blank 3lb shown in FIG. 5B is obtained.
By using, as the above die, one capable of 10 forming a projection of a rectangular contour in cross-section (hereinafter simply referred to as a rectangular projection) on the opposite side of the blank upon conducting the above end face setting or sizing, it is possible to provide the second blank 3lb with a rectangular projection 32 on the opposite end and a tapered conical surface 33 formed on a basal portion of the rectangular projection 32. This makes it possible to easily perform extrusion forming in a second step.
(2) Second step: The second blank 3lb, which has been subjected to the end face setting or sizing, is inserted in a die and a spindle portion having a rectangular cross-section is then extrusion-formed by a punch. This extrusion forming provides a first preform 31c in which, as depicted in FIG. 5C, a cylindrical head portion 34 and a spindle portion 35 of a rectangular cross-section are connected together at the tapered surface 33.
(3) Third step: The first preform 31c is punched. By this punching, the head portion 34 of the first preform 31c is gradually tapered in an upward direction as viewed in FIG. 5D, whereby the top portion 34 is formed into a frustum of a right circular cone to obtain a second preform 31d.
(4) Fourth step: The head portion 34 of the second preform 31d is punched. As shown in FIG. 5E, a flange-shaped disc portion 36 and a projection 37 centrally defining a slot or a square recess therein are formed by this punching so that a third preform 31e is formed. At the same time, a tapered wall 33 (see FIG. 5B) is also formed on a stepped portion 41. The third preform 31e has the shape that the spindle portion 35 of the rectangular cross-section and the flange-shaped disc portion 36 extend in continuation with each other.
(5) Fifth step: Teeth 38 of a ratchet gear are punched in an outer peripheral portion of the flange-shaped disc portion 36 of the third preform 31e, whereby a formed product 40 shown in FIG. 5F is obtained as intended.
Such a conventional production process of a webbing take-up spindle, said process making use of cold forming, is however accompanied by problems as will be described next.
In the fifth step of the production process described above, the ratchet teeth are punched in the outer peripheral portion of the flange-shaped disc portion 36 of the third preform 31e. As a result, secondary fracture surfaces or fractures 39 maybe formed in tooth walls of the teeth 38 as shown in FIG. 6, leading to potential problems such that the tooth walls could have poor properties and the service life of a punching die could be shortened